Microphone



Nov. 14, 1939. G W. SUTTON 2,179,733

MICROPHONE Filed Nov. 25, 1955 3 Sheets-Sheet l 33 Fzg. I.

I6 I I3 30 7 II I l II ii 28 I we\ 26 27 g n 12 5 24 i f, 7 6 a M (fl/1 NOV. 14, 1939. G w SUTTON 2,179,733

MICROPHONE Filed Nov. 25, 1955 5 Sheets-Sheet 2 64 Fzg, 4. K

Q 74 5M W Nov. 14, 1939. G w SUTTON 2,179,733

MICROPHONE Filed Nov. 25, 1935 3 Sheets-Sheet 3 Patented Nov. 14, 1939 NITED STATES TENT GFFHQE.

MICROPHONE Appli In Great Britain 4 Claims.

The present invention relates to microphones or transmitters such as are used in telephone, public address and sound recording systems and more particularly to improvements in the form t and construction of microphones or transmitters of the differential type in which carbon or other conducting granules are used.

By a microphone or transmitter of the differential type is to be understood one which has two paths for current, a movement of the diaphragm causing the resistance of one of the paths to be increased as the resistance of the other path is decreased. In the usual differential transmitter a moving electrode which is coupled to the diaphragm is situated between two fixed electrodes. It is also feasible to have three fixed electrodes, the pressure of the granules being varied by an element coupled to the diaphragm.

According to one feature of the invention, a differential microphone or transmitter is constructed so that with a proper filling of granules in the granule chamber a substantial head of granules acts on the active face or faces of each of the three electrodes at all positions of the transmitter. This construction helps among other things to ensure that the two halves of the transmission system corresponding to the two current paths referred to in the preceding paragraph are maintained substantially inbalance under all working conditions. With a differential microphone or transmitter it is important that a sufficient degree of balance should be maintained, as a lack of balance results not only in a decreased efficiency of conversion from sound waves to alternating currents but also in a diminution of that freedom from the production of even harmonies and from trouble due to a noisy source of microphone current which should result from push-pull operation. The terms microphone and transmitter are used in a wide sense, and are intended to cover a transmitter element with or without parts not concerned with the invention such as a holder or mouthpiece.

In applying the invention to a differential microphone or transmitter of the kind comprising two fixed electrodes and a moving electrode, it is preferably arranged that the maintainance of a substantial head of granules on the active face of each fixed electrode at all positions of the microphone or transmitter is due to the use of a construction wherein the active face of each fixed electrode stands away from the end of the granule chamber on which the electrode is supported and wherein a space for the accommodation of granules exists around the body of the cation November 25, 1935, Serial No. 51,501

November 27, 1934 electrode and/or around a member supporting the electrode and projecting into the granule chamber. A construction may be used wherein a diaphragm to which the moving electrode is coupled has a central portion consisting of a cone of thin material wherein the apex of this cone is attached to the moving electrode by a rod or pin which passes through a hole in a fixed electrode lying between the diaphragm and the 7 moving electrode, and wherein the apex is situated within a depression in a wall of the granule chamber, this depression having its counterpart inside the granule chamber in a protuberance on which the said fixed electrode is mounted. This construction has the advantage that it allows the rod or pin to be short and rigid and yet provides for the immersion in the granules of the fixed electrode concerned.

In certain contemplated embodiments of the invention comprising microphones or transmitters of the kind having two fixed electrodes and a moving electrode, the overall internal dimensions of the granule chamber in directions transverse to its length are considerably greater than would be necessary just to accommodate the fixed and moving electrodes, and according to a subordinate feature of the invention in cases where such a construction is employed an insulating barrier is provided round the moving electrode to prevent or minimise the flow of current through the granules in paths not including this electrode and in paths the resistance of which is not effectively varied by movement of this electrode. The insulating barrier may be a partial one allowing granules to pass freely from one section of the granule chamber to the other, or it may be a complete barrier, in which case care must be taken to ensure that in spite of the presence of the barrier a substantial head of granules acts on the active faces of the moving electrode at all positions of the microphone or transmitter.

The invention is applicable to a differential microphone or transmitter of the kind in which a modulating member attached to a diaphragm and situated between two fixed electrodes is not itself an electrode but acts to vary the resistance of the current paths between the two fixed electrodes mentioned and a third fixed electrode. In

a contemplated embodiment of the invention encompasses the other two electrodes and the modulating member.

A microphone or transmitter according to the invention may be used as an ordinary subscribers or operators transmitter'in a public or private telephone system, and, owing to the sensitivity resulting from the use of the diiferential principle, it may also for example be used in cases Where it is desired to operate a loudspeaking receiver or receivers without the aid of an amplifier or where it is desired to render it unnecessary for the source of sound to be situated close to the transmitter.

A number of specific embodiments of the invention will now be described with reference to the accompanying drawings. The invention is not limited to these embodiments, which are described by way of example only. Fig. 1 of the drawings shows a cross-sectional view of a capsule or inset transmitter according to the invention, and Figs. 2, 3, 4 and 5 show cross-sectional view of other capsule or inset transmitters according to the invention. With the exception of Figs. 1 and 5, each figure shows only part of the transmitter concerned. For convenience in laying out the drawings, the transmitters in Figs. 2-4 are shown lying on their backs although this would of course not be the usual position when in use.

Referring firstly to Fig. 1, the capsule transmitter shown has a moving electrode I situated between two fixed electrodes 2 and 3. It is of course intended that the two microphone elements formed by the variable resistance path between the moving electrode and one fixed electrode, and the variable resistance path between the moving electrode and the other fixed electrode, shall be connected in the usual manner to give a push-pull effect.

The transmitter has a thin aluminium alloy diaphragm consisting mainly of a central portion 4 of conical form, this portion being surrounded by a flat annular portion 5 to which it is connected by a further portion 6 having the form of a frustum of a cone. The periphery of the diaphragm is clamped to the main casing I of the transmitter by a spun brass ring 8. The outer face of the diaphragm is stove enamelled to minimise the possibility of corrosion. The casing 1 is of brass, and a brass guard plate 9 which fits tightly on the ring 8 and is carried thereby serves to protect the comparatively fragile diaphragm from damage. The guard plate has apertures I0 to allow sound waves to reach the diaphragm.

Attached to the apex of the central conical portion 4 of the diaphragm is one end of a light aluminium alloy rod or pin H, the other end of this rod being attached to the moving electrode I by means of a small aluminium alloy nut I3. Although the rod is shown in crosssection, the usual shading has been omitted in order to avoid confusion of the drawings. The parts of the transmitter are designed in such manner as to enable the rod I I to be as short and rigid as is possible compatible with a permissible weight. The rod I l the diaphragm, and the casing I serve to provide the necessary electrical connection to the moving electrode l. The moving electrode is a stiff disc of carbon having a hole drilled in its centre, the requisite rigidity compatible with a permissible weight being obtained by making the disc thick near the centre and thin at the edge. The diameter of the disc is about one fifth that of the free portion of the diaphragm, and the disc vibrates substantially as a whole at the frequencies which are important for the transmission of articulate speech. The shape of the disc tends to minimise the flow of current through the granules in paths the resistance of which is not effectively varied by movement of the disc.

In the quiescent condition, the moving electrode I is equidistant from the fixed electrodes 2 and 3, which are carbon electrodes of annular form. It is always totally immersed in the granules, which are of carbon. The granules are not shown in the drawings, but a normal filling of granules occupies a sufficiently large portion of the available volume of the granule chamber to ensure that the active faces of all three electrodes remain totally immersed in granules at all positions of the transmitter. The surfaces of the fixed electrodes which face the moving electrode (1. e. the active faces) are shaped in such a manner that the opposing surfaces of the fixed and moving electrodes are substantially parallel. The rod ll passes through the hole in the centre of the front fixed electrode 2,

a felt washer I4 surrounding the rod to prevent w leakage of granules from the granule chamber. The felt washer is held in position by a further washer l5 and a sprung ring It. The further washer I5 is an insulating washer made of a synthetic resin product. of the back fixed electrode 3 serves to provide the passage through which the granules are inserted into the granule chamber on the com pletion of the assembly of the transmitter, and

also serves to make the mechanical impedance to displacement of the moving electrode more nearly the same for similar displacements in either direction and to maintain the symmetry of the two current paths through the transmitter. Since the portion of the rod II lying within the hole in fixed electrode 2 is appreciably smaller in diameter than the hole, both the holes in the fixed electrodes serve to provide room for a small quantity of granules and thus assist to some extent in that immersion of the fixed electrodes which is a feature of the transmitter.

The granule chamber is carried by the casing I, the cylindrical brass wall ll of the chamber being insulated from the casing l by insulating material l8. In manufacturing the transmitter, the material I8 is pressed, moulded, or squirted into the correct form and position. The material is locked in position in the casing l by rivet-shaped portions such as 19, and three projecting portions 29 spaced apart with respect to the axis of the capsule lock the granule chamber in position. The material may conveniently be, a material such as ebonite or a cellulose acetate compound or a synthetic resin product. The front wall 2! of the granule chamber is of brass and is fixed to the cylindrical wall I! by the spun over edge 22 of the latter. The front fixed electrode 2 is mounted directly on the front wall so that the cylindrical wall I! serves as one terminal of the transmitter in the way that the casing I serves as another terminal. The back fixed electrode 3 is carried by a brass member 23 which in turn is carried by a member 24 composed of insulating material. The member 24 forms the rear wall of the granule chamber, and is fixed to the cylindrical wall I! by the spun over edge 25 of the latter. The member is secured against rotation by a punched out portion 34 of the wall I! which The hole in the centre engages a corresponding slot in the member. The brass member 23 has a hollow stub 26, and the spun over edge 21 of this stub serves to fixv the member 23 securely to the member 24 and also forms the third terminal of the transmitter. A screw 23 closes the passage through which the granules are inserted into the granule chamber on the completion of the assembly of the capsule. The fixed electrodes 2 and ii are situated in cup-shaped receptacles formed in the members 2i and 23, the spun over sides of the cups holding the electrodes firmly in position. Air passages 29 and 36 pass through the members 23 and 2d, and a felt washer l2 pervious to air serves to prevent leakage of granules via these passages. The function of the passages is to ensure that the air pressure behind the diaphragm and in the granule chamber remains substantially the same as that of the outside air.

It will be seen from the drawings that both the internal diameter and internal length of the granule chamber are greater than would be necessary just to accommodate the fixed and moving electrodes with their active faces at the appropriate distance apart (e. g. the diameter is about one and three quarter times that of the electrodes) and that the two. fixed electrodes are arranged to protrude into the granule chamber, the elfect being to immerse these electrodes in the granules. The form of the member 2i is such that it allows the rod H to be short and rigid and yet provides for the immersion of the front electrode 2 and a sumcient air-space behind the diaphragm. The immersion of the electrodes operates to the end that a substantial head of granules acts on the active face or faces of each of the three electrodes at all positions of the transmitter.

The interior walls of the granule chamber (including the surfaces of pin El and nut l3) and the exposed surfaces of the fixed electrodes other than the active faces are covered with a layer of insulating varnish or enamel.

The transmitter is of a size suitable for insertion in the transmitter container of a handmicrotelephone or in a holder of similar dimensions. The member 83 is a form of spring ring and'its purpose is to force the transmitter against the mouthpiece part of a holder into which it is inserted. The punched out portions or projections 35 of the casing I serve to secure the spring ring. The projections are four in number, three (including the one shown at the top in the figure) being spaced 129 apart with respect to the axis of the transmitter and one (shown at the bottom in the figure) being midway between two of the others. The three equally-spaced projections are spaced from the flange of the case while the fourth projection abuts it. The edge of the central hole of the spring ring 33 is cut away in three places corresponding to the three equally spaced projections, but each of the cuts extends for a considerable distance round the periphery of the hole. In assembling the ring in position on the it is pressed against the flange of the cos-in with the cuts coinciding with the four projections (the cuts are sufficiently long to enable .318 odd projection to enter the same cut as one of the adjacent equally spaced projections) an" then rotated until the end of one of the cuts abuts the odd projection. The spring ring has series of holes 36 out in it to decrease its stiffness. In so far as the terminals of the transi'nitter are concerned the ring where provided' is looked upon as forming part of the casing i.

It will be realised that the three terminals formed by the surfaces Eli and 32 of members 33 and il and the spun over edge 21 of stub 26 are of such a form that contact between them and contact springs in a casing adapted to receive the transmitter is unaffected by rotation of the transmitter.

The passage through which the granules are inserted may alternatively be closed by a cap permanently fixed in position by a spinning operation. It should be noticed that where this is done a capsule transmitter is obtained which cannot be tampered with by undoing screws or screwed parts.

in describing the embodiments shown in the remaining figures, only points in which the construction differs from that of a previously described embodiment will be dealt with in detail. For the sake of simplicity, the granular filling of the granule chamber is not shown in any of the figures.

Referring now to Fig. 2, this shows a capsule transmitter the construction of which only differs materially from that of the capsule transmitter shown in Fi l in that an insulating barrier is provided around the moving electrode. The barrier is formed by an annular portion of a disc or membrane 3? of. very thin and flexible insulating material such as Swiss silk. The disc or membrane 5? a small hole at its centre, and is threaded on the screwed portion of the rod or pin H which couples the moving electrode to the diaphragm. The moving electrode is composed of two separate carbon discs E2 and 43 between which the central portion of the disc 3'! is clamped. The periphery of the disc 31 is clamped between two brass members 38 and 39 which together form the cylindrical wall of the granule chamber and correspond to member IT in the capsule transmitt-r of Fig. 1. As shown in the figure, member 35 is held in position by a spun over edge as of member 38, but it could equally well be arranged to screw into member 38. Similarly, of course, member 2d could be arranged to screw into member 39 instead of being held by a spun over edge of the latter. The unclamped annular portion of the disc ill is slack and not taut, and this, in conjunction with the thinness and flexibility of the material of which the disc compose enables the pressure due to a head of granules on one side to be readily transmitted to granules on the other side. By suitably filling the two halves of the granule chamber with.

granules it can therefore be arranged that a substantial head of granules acts on both faces of the moving electrode at all positions of the transmitter. The barrier formed by the disc serves to prevent the flow of current through the granules in paths not including the moving electrode to minimise the ilow of current in paths the resistance of which is not effectively varied by movement of this electrode.

In assembling the transmitter, the correct quantity of granules for the half of the granule chamber through whi .1 rod it passes is placed in the said. half before the parts t2, 3?, 43, I3, and are fitted and fixed in position.

In tl e capsuletransmitter Fig. 2 just described, the disc ill may take the form of a comparatively stifi annular washer which is of insulating material and which is supported only by being clamped at its outer edge between members 38 and Bi The annular washer may conveniently be made of a synthetic resin product. The moving electrode in this case is in one piece as in the capsule transmitter of Fig. 1, and the central hole of the annular washer is appreciably larger than the electrode so that the annular gap around the moving electrode between the electrode and the washer is sufiiciently wide to allow the free passage of granules. The annular gap is however not wide enough seriously to impair the effect of the barrier formed by the washer in so far as confining current flow to desired paths is concerned.

Referring now to Fig. 3, this shows a capsule transmitter in which the granule chamber is internally equivalent to that of the capsule transmitter described in the preceding paragraph but in which the partial barrier around the moving electrode is formed by a fin 44 of insulating material which is in one piece with the material l8 which insulates the cylindrical brass wall l! of the granule chamber from the casing l. The fin is linked with the main body of insulating material via circular holes 4] in the wall of the granule chamber. In manufacturing the transmitter, the insulating material is pressed, moulded or squirted into the correct form and position.

Referring now to Fig. 4, this shows a capsule transmitter having a granule chamber in which a complete insulating barrier around the moving electrode is formed by an auxiliary diaphragm 55 which is fairly stiff and may for example be of mica or of metal covered with a layer of insulat ing varnish or enamel. The diaphragm 55 is adapted to play an important part in controlling the movement of the vibrating system of the transmitter, and makes it possible for the main diaphragm to be flexibly mounted at its edge (1. e. to be a floating diaphragm). The diaphragm 55 has a small hole at its centre, and is threaded on the screwed portion of the rod or pin II. The moving electrode is composed of two separate carbon members 57! and 58 between which the central portion of the diaphragm 55 is clamped. Each of the two carbon members consistsof a ring of carbon in which a channel 6! has been cut in order to make the vibrating system as light as possible. The periphery of the diaphragm is clamped between the two brass members 38 and 39 which together form the cylindrical wall of the granule chamber. The member 2 3 of insulating material is clamped to member 39 by a clamping ring 56. In the case of the figure now being referred to, it has been deemed advisable to indicate the various layers of insulating varnish or enamel inside the granule chamber (such as 59, 60, 62). It will be seen that the two annular active faces of the composite moving electrode stand away from the corresponding faces of the diaphragm 55 so that spaces for the accommodation of granules exist around the body of the electrode. -This construction ensures that an adequate head of granules acts on the said' active faces at all positions of the transmitter. The hole 63 in the casing l serves to maintain equal air pressures on both sides of the main diaphragm. The main diaphragm G l is of an aluminium alloy and is a floating diaphragm having its outer edge lightly damped by packs of paper washers 85. A good electrical connection between the diaphragm 65 and the casing l is provided by a flexible metal spider 82. The diaphragm may be constructed in known manner of two concave shells of thin light metallic material placed concentrically with the concave sides facing each other, the construction being such that the edges of the shells clamp between them a small aluminium alloy nut l3.

a thin disc or annulus of metal foil the outer edge of which is clamped to the casing 1.

Referring now to Fig. 5, this shows a capsule transmitter in which all three electrodes are fixed electrodes, the resistance of the current paths between the two similar electrodes 2 and 3 and the other electrode 66 being varied during the operation of the transmitter by a modulating member 67 attached to the diaphragm by an aluminium alloy rod or pin I i. In effect, the modus1 lating member 67 and electrode 66 together perform the functions performed by the moving electrode in the previously described embodiments. The modulating member 6'! is of insulating material, and is held in position on rod 1 l by Both the member and all three electrodes are solids of revolution about the axis of the capsule. Both faces of the aluminium alloy diaphragm are stove enamelled to minimise the possibility of corrosion. The enamel also serves to insulate the diaphragm from the brass casing T which in this embodiment is electrically connected to electrode 2. It will be seen that electrode 66 is a composite one consisting of two parts 68 and $9. The active faces "i6 and "ii of electrodes 2 and 3-are parallel to the corresponding active faces 72 and E3 of electrode 86. The exposed surfaces of the electrodes other than the active faces, and the interior walls of the granule chamber where of metal (including the surfaces of pin it and nut l3) are covered with a layer of insulating varnish or enamel 14. A normal filling of granules occupies a sufiiciently large ortion of the available volume of the granule chamber to ensure that a substantial head of granules acts on the active face or faces of each of the three electrodes at all positions of the transmitter. The felt washers l2 and I4 correspond to the washers similarly designated in Fig. 1 and serve to prevent leakage of granules from the granule chamber. The cylindrical brass wall 75 of the granule chamber is carried by the casing F but is insulated therefrom by insulating material l8. In manufacturing the transmitter, the material 18 is pressed, moulded, or squirted into the correct form and position.

The front wall 76 of the granule chamber is of brass and is fixed to the casing 1 by a spun over edge ll of the latter. The front electrode 2 is mounted directly on the wall '16. The back electrode 3 is carried by a brass member 23 which in turn is carried by a member 24, composed of insulating material. The member 24 and the parts 553 and 5? of electrode 66 are clamped in position in thebrass member 15 by a metal clamping ring 56. The passage through which the granules are inserted is closed by a metal cap 8!.

The terminals for electrodes 2, 3 and 66 are formed by the surfaces l8, l9, and 80 respectively, and are hence of such a form that contact between them and contact springs in a casing adapted to receive the transmitter is unaffected by rotation of the transmitter.

I claim:

1. Ina differential transmitter, a diaphragm, a microphone chamber having resistance material therein, a pair of fixed electrodes supported on :opposite walls of said chamber, a movable electrode located between said fixed electrodes and connected to said diaphragm, a current path ex- 7 tending between said movable electrode and both of said fixed electrodes through said resistance material, and a barrier in said chamber extending around said movable electrode out of contact therewith for minimizing the flow of current through said resistance material in the paths not including said movable electrode and in paths in which the resistance of the material is not effectively varied by the movement of said movable electrode.

2. In a differential transmitter, a diaphragm, a microphone chamber having resistance material therein, a pair of fixed electrodes supported on opposite walls of said chamber, a movable electrode located between said fixed electrodes and having a connection with said diaphragm, a current path extending between said movable electrode said fixed electrodes and through said resistance material, and a barrier extending around the periphery of said movable electrode for minimizing the flow of current through said resistance material in paths not including said movable electrode, said barrier being constructed of insulating material, and means for supporting said barrier on the chamber walls so that it will be out of engagement with said movable electrode.

3. In a differential transmitter, two stationary electrodes, a granule chamber filled with granules, end members on said chamber each having a raised portion extending inward toward the center of the chamber supporting said electrodes, an intermediate element, means for supporting said element between said electrodes, said intermediate element comprising a movable third electrode, the over all internal dimensions of the granule chamber in directions transverse to its length being greater than necessary to accommodate said fixed and movable electrodes, so that the raised portions of the end members and said element are completely covered by the said granules regardless of the position of the transmitter, and an insulating barrier mounted in the chamber around but out of engagement with the periphery of said movable electrode for minimizing the current flow through the carbon granules in paths in which the resistance is not effectively varied by a movement of the movable electrode.

4. In a difierential transmitter, two stationary electrodes, a granule chamber, end members on said chamber each having a raised portion extending inward toward the center of the chamber supporting said electrodes, an intermediate element, means for supporting said element as a movable third electrode between said other electrodes, said chamber of such dimensions that when properly filled with carbon granules the exposed surfaces of said electrodes and said element of the raised portions of said end members are completely covered by the said granules regardless of the position of said transmitter, and a partial insulating barrier mounted in said chamber around but out of engagement with the periphery of said movable electrode to form two sections for said chamber while permitting said carbon granules to pass freely from one section of said chamber to the other.

GEORGE WILLIAM SUTTON. 

